The fusion of deuterium (D) and tritium (T) atoms has been proven in the laboratory to produce the highest energy gain at the "lowest" temperatures.
Although different isotopes of light elements can be paired to achieve fusion, the deuterium-tritium (DT) reaction has been identified as the most efficient for fusion devices. ITER and future devices will use the hydrogen isotopes deuterium and tritium to fuel the fusion reaction.
Deuterium can be distilled from all forms of water. It is a widely available, harmless, and virtually inexhaustible resource. In every litre of seawater, for example, there are 33 milligrams of deuterium. Deuterium is routinely produced for scientific and industrial applications.
Tritium is a fast-decaying radioelement of hydrogen which occurs only in trace quantities in nature. It can be produced during the fusion reaction through contact with lithium, however: tritium is produced, or "bred," when neutrons escaping the plasma interact with lithium contained in the blanket wall of the tokamak.
Lithium from proven, easily extractable land-based resources would provide a stock sufficient to operate fusion power plants for more than 1,000 years. What's more, lithium can be extracted from ocean water, where reserves are practically unlimited (enough to fulfill the world's energy needs for ~ 6 million years).
Global inventory for tritium is presently around twenty kilos, which ITER will draw upon during its operational phase. The concept of "breeding" tritium within the fusion reaction is an important one for the future needs of a large-scale fusion power plant.
A fusion reaction is about four million times more energetic than a chemical reaction such as the burning of coal, oil or gas. While a 1,000 MW coal-fired power plant requires 2.7 million tonnes of coal per year, a fusion plant of the kind envisioned for the second half of this century will only require 250 kilos of fuel per year, half of it deuterium, half of it tritium.
Only a few grams of fuel are present in the plasma at any given moment. This makes a fusion reactor incredibly economical in its fuel consumption and also confers important safety benefits to the installation.